April 20 (UPI) -- Veteran NASA astronaut Don Pettit celebrated his 70th birthday as he returned to Earth from a seven-month stay at the International Space Station, during which he completed Expeditions 71 and 72. Pettit is the oldest currently serving astronaut in the U.S. and has flown to space four times, logging some 590 days in orbit throughout his career. He is the second-oldest American astronaut to visit space after John Glenn 's famed 1998 mission at the age of 77. "The feeling of